The purpose of this project is a detailed investigation at the molecular level of phosphoglucose isomerase and phosphomannose isomerase, both of which have fructose-6-phosphate as the product of their forward reaction, with the aim of obtaining specific and quantitative information with respect to similarities and differences in their mechanism of action. Both enzymes form the identical product in one direction of their catalyzed reaction. Since the products formed in the other direction distinguish themselves only by a different steric configuration of one hydrogen at one carbon, which might be considered the least variation necessary for two distinguishable reactions, the two reactions studied present an ideal object for the study of enzyme specificity. The aim, therefore, is to elucidate the structural features of the two enzyme proteins which enable them to distinguish between these very slight differences in the substrate(s) to be attacked. The approach to the problem includes isolation of both isomerases from several sources, their chemical and physical characterization, a thorough kinetic analysis of their respective catalyzed reactions including the use of transition-state-analogues, elucidation of their active site structures, investigation of the conditions required for their active conformations and evaluation of factors controlling their quarternary structures, and the synthesis of active-site directed affinity labels to form covalently linked ES-compounds. Also, a joint project on studies of the three-dimensional structure of phosphoglucose isomerase is under way involving determination of the total amino acid sequence at Riverside and x-ray crystallography work at the University of Bristol.